P. T. Olsen

NBS determination of the fine-structure constant, and of the quantized Hall resistance and Josephson frequency-to-voltage quotient in SI units

Results of US National Bureau of Standards (NBS) experiments to realize the ohm and the watt, to determine the proton gyromagnetic ratio by the low-field method, to determine the time dependence of the NBS representation of the ohm using the quantum Hall effect, and to maintain the NBS representation of the volt using the Josephson effect, are appropriately combined to obtain an accurate value of the fine-structure constant and of the quantized Hall resistance in SI units, and values in SI units of the Josephson frequency-to-voltage quotient, Planck constant and elementary charge. >

A measurement of the NBS electrical watt in SI units

The National Bureau of Standards (NBS) electric watt in SI units to be: W/sub NBS//W=K/sub W/=1-(16.69+or-1.33) p.p.m. The uncertainty of 1.33 p.p.m. has the significance of a standard deviation and includes the best estimate of random and known or suspected systematic uncertainties. The mean time of the measurement is May 15, 1988. Combined with the measurement of the NBS ohm in SI units: Omega /sub NBS// Omega =K/sub Omega /=1-(1.593+or-0.022) p.p.m., this leads to a Josephson frequency/voltage quotient of E/sub J/=E/sub 0/(1+(7.94+or-0.67) p.p.m.) where E/sub 0/=483, 594 GHz/V. >

Monitoring the mass standard via the comparison of mechanical to electrical power

An ongoing absolute watt experiment that shows the promise of being able to monitor the stability of the kilogram standard to better than 0.05 p.p.m. is discussed. The theory is presented, and the latest improvements to the experimental apparatus are briefly described. >

A Noncontacting Magnetic Pickup Probe for Measuring the Pitch of a Precision Solenoid

The magnetic-field gradients produced by a current sequentially activating a few turns of wire of a precision solenoid are used to measure its pitch. The position of the activated portion of wire can be resolved to 0.1 ?m. Preliminary results are found to be in agreement with an earlier measurement using a contacting probe to within the uncertainty of the latter determination. This new technique reduces many of the difficulties associated with conventional pitch measuring schemes and at the same time provides a method of obtaining increased accuracy.

The NIST watt balance: progress toward monitoring the kilogram

The National Institute of Standards and Technology (NIST) watt balance is an experiment to compare measurements of the watt using electrical references (volt, ohm) to those using mechanical references (length, time, mass). A coil within a radial magnetic field has a dual use of: (1) generating a voltage by moving at some velocity to calibrate the magnetic flux density, and, (2) generating a force with electrical current to balance the gravitational force of a mass. This experiment has had several improvements made to it in the last year. These include the incorporation of three-laser interferometry and a refractometer to improve the velocity measurements, temperature control and coil rotation damping to reduce drifts and stabilize laser and mechanical alignments, and a gravimeter to determine local gravity. Systematic errors and scatter in long-term measurements have been greatly reduced in the last year, but statistically significant deviations relative to within-run uncertainty still persist. The source of these deviations has not yet been identified. Recent within-run standard deviations are generally near 0.1 pW/W, which is the target precision of this present design.

The NBS Absolute Ampere Experiment

We have constructed a current balance with superconducting field coils for the realization of the SI ampere by comparing mechanical to electrical work. The estimated ultimate accuracy of the realization is 0.1 ppm. We describe and present preliminary results obtained with a room temperature version of the apparatus.

The Realization of the Ampere at NBS

We present a method for the realization of the ampere based on Faradays induction law and using a modification of the classic Pellat balance. A preliminary apparatus has been constructed and initial measurements have been obtained. This balance is also compared with a balance similar to one proposed earlier.

Report on the New NBS Determination of the Proton Gyromagnetic Ratio

We describe a new measurement of the proton gyromagnetic ratio in H2O, ¿¿p, now in progress at NBS, including the construction of a single layer precision solenoid, the measurement of its dimensions by an inductive technique, and our latest dimensional measurement results. We also discuss other improvements made since our last ¿¿p determination.

Standard Cell Calibration via Current Transfer

The EMFs of standard cells are now being transferred between laboratories over a 1?-km cable with a precision of 4 parts in 108 to provide an instantaneous comparison of the 2e/h and ?p? experiments being carried out at the two facilities. This is accomplished by transferring a constant current that produces a 1-V drop across standard resistors located at both ends of the cable.

Nuclear magnetic resonance-based current-voltage source

A solenoid geometry for producing magnetic fields is introduced. A 1-A current has been stabilized using nuclear magnetic resonance techniques. A pair of tandem solenoids produces two uniform magnetic fields in opposite directions, and these fields are not affected by external magnetic shielding. The current and background field are controlled to within 0.1 ppm over an 8-h period. >